Ultimate LRS Mini


There are several modules on the market that can be used with Ultimate LRS, including the Hobbyking OrangeRX modules.

However it’s also possible to build yourself an Ultimate LRS Mini. The idea was to propose a simple schematic, and let the community design PCB based on the schematic. The results are above all expectations, and you can find on this page 8 different designs (actually a little more as some design have a different PCB for TX and for RX).

Notice that IT Luxembourg doesn’t sell any hardware, no PCB, components or modules.


One the 8 ULRS Mini designs

Ultimate LRS Mini Reference Schematic


The reference schematic was based on these principles :

  • Must be easy to implement, copy, improve
  • Small, can fit in the transmitter
  • Cheap :
    • Arduino pro mini 5V = $4.12 (link), or $2.11 (link)
    • RFM23BP = $10.77 (link)
  • Arduino mini based :
    • Integrated voltage regulator, decoupling capacitors, power on LED, reset switch, accessible pins
    • Widely supported platform, fifth version of the Arduino mini
    • The Arduino mini is one of the smallest Arduino boards, with a size similar to the RFM23BP itself
  • Powered at 5V
    • Ensure higher power for the RFM23BP module
    • Within specs of the atmega328 for 16 MHz operation (avoids eeprom issues)
  • Simple wiring
    • Arduino mini IO pins facing exactly the RFM23BP pins have been selected, can easily be implemented on a breadboard or prototype board.
    • No need for NIRQ pin, only 4 pins + power.
  • Maximum safety
    • Atmega328 16 MHz powered at 5V (3.3V would be outside specifications)
    • All communication pins to RFM23BP, PPM, Serial port, RSSI are resistor protected, which ensures a low current and correct voltage on these pins.
  • Filtered RSSI
    • No need for an external RSSI lowpass filter
  • 7 remaining digital IO pins

Current version

Schematic for version 1.04

2016-06-19 06_57_23-1 Schematic - C__Users_Benoit_Documents_eagle_ULRS mini_ULRS mini 2.sch - EAGLE

ULRS Mini Reference Schematic V1.04


  • The FTDI DTR is connected to the RST pin through a capacitor. This will work, an alternative option would be to connect it directly to the Arduino pro mini ‘DTR’ pin (marked as GRN on the Arduino pro mini boards). However on the Eagle library used above this pin was not available.
  • Using linear regulators gives a very clean power supply, but they warm easily if powered from the battery. A drop-in switching regulator replacement for LM7805 could be this one. It could also replace the LM317 but be careful as the pinout is different.
  • The schematic is volutarily kept extremely simple and can be made on a breadboard or prototype board. It can be reused as is or modified in your own products.
  • Not shown on the schema, but the buzzer can be connected on Arduino digital pin 8.
  • Some changes are planned on this schematic :
    • When I did this schematic, I thought the atmega could only support 5.5V, and that’s why there’s a 5V and a 6V power supply… which isn’t ideal.So the new design will have only one power supply at 6V (which is supported by both atmega and RFM), and probably a switching power supply as the linear ones warms too much in practice.Powering everything at 6V brings some attention points :
      – if I2C devices are connected we must ensure they can support this voltage. Notice that on an I2C port we can’t put resistors on the SDA and SCL pins.
      – same for serial ports (but with the resistors there’s no risk)I considered adopting a real voltage shifter for the RFM, but until now nobody complained about a burned RFM in the ULRS Mini boards : the resistors seem to be sufficient and limit the current to an extremely low value. Notice that some implementations such as the Sami board have a full voltage shifter chip, which is perfect, but the reference schematic’s objective is to keep it as simple as possible while staying completely reliable.On the Arduino mini library used, the I2C pins are not accessible, so I’ll try to find another better library that includes all the Arduino mini pins. I know that not many users have implemented I2C devices, except the Adafruit 16 channels extender, but this port will be very useful soon for the diversity features : you’ll be able to use any two ULRS boards and connect them by the I2C port to have a full diversity system.
    • Edit : recommended voltage is 5V, at 6V the RFM tends to warm too much.

Ultimate LRS Mini modules

Several users have made ULRS Mini modules based on the reference schematic.

You can choose any of these options, but check out their differences.

The Monkey Board : a smart through-holes board that fits into the RC transmitter

ULRS Monkey board TX (left) and RX (right) modules

A design based on ULRS Mini schematic by rcgroups user neo_04, full infos here and here. Two different boards, one for TX, one for RX.

  • TX:
    • Fits inside empty Frsky Case
    • Pins for FTDI, button, buzzer
    • Pins to suit bluetooth module
    • Pins to suit teensy telemetry mod
    • Switching regulator running everything at 5v.
    • With shield on RF module, bluetooth and teensy, everything fits perfectly inside Frsky case with lid on,
      just cut a hole to put the antenna through
    • You choose how to connect the antenna, eg. 5cm pigtail sma lead etc.
    • Link to PCB order page for TX : http://dirtypcbs.com/store/designer/…7/ulrs-mini-tx
  • RX:
  • Build instructions


Through-holes board that fits into the RC transmitter : the Starfish board

  • Rcgroups user 007trains has made this PCB for you :

  • Features :
    Small size 3.9cm x 5cm
    Onboard TO-220 regulator
    Uses through hole components for simple build
    Transmitter JR slot compatible connections
    Header pin for serial: gnd,rx,tx,5v
    Header pin for button and buzzer: gnd,button,buzzer,5v
  • Link to original post
  • Link to PCB order page

The Joey board : a small through-holes board

  • This is another design from the Australian ULRS user yak-54 who made the Wombat board
  • A Joey is an infant marsupial, such as a kangaroo (wikipedia), a good name for the small Australian board…

  • It fits perfectly into the $5 case sold by Banggood but be careful : it’s not intended to be plugged into an RC TX (it doesn’t have a connector in the right place for this). Have a look at the Starfish or Monkey boards if you want to plug into a RC TX.

The Wombat board : a simple through-holes board

Version 1

Very similar to the Spongeboard, but smaller, this design from Australian rcgroups user yak-54 looks very nice :

Version 2


ULRS Mini without SMD components : the Sponge Board

User ‘ljakob’ from rcgroups did a PCB for Ultimate LRS Mini that can be ordered right now. It only uses through hole components and is very easy to build :


2016-06-13 06_22_48-Dirt Cheap Dirty Boards

  • Link to the the component list and schematic:
  • Errata : the PPM is connected to the atmega A0 pin and not A3 as shown on this schematic. Notice that the firmware does support this board despite this difference from the reference schematic.
  • Component list:
    • R1 to R4 : 3.3k resistors, 1/4 watt
    • R5, R6 : 220 ohms resistors, 1/4 watt
    • R7 : 2.2k resistor, 1/4 watt
    • R8, R9 : 1k resistor, 1/4 watt
    • C1 : 150µF capacitor, 16V or more. Value is not critical, for example this 100 µF capacitor of this one will work. (not mandatory)
    • C2 : 100pF capacitor (example) (not mandatory)
    • C3 : 100nF capacitor (example) (not mandatory)
    • C4 : 100nF capacitor (example)
    • LED1, LED2 : standard LEDs, but not all brands and colors will give good results, these ones give a very good luminosity.
    • SMA connector : this one or this one (angled at 90°)
    • Pin headers : for example this one.
    • RC transmitter connector : use this.
    • Remark :
      • For resistors you may be interested by a pack of 500 resistors (link1, link2)
      • Same for capacitors : link1, link2
  • No voltage regulator included, must be powered at 5V.
  • Be careful to orient correctly the arduino pro mini and the RFM23BP, refer to the photo.
  • Notice that it will even work without the capacitors C1 to C3 (C4 is still required), which makes it one of the simplest ULRS Mini to build.
  • The LEDs can be any common model, but as their brightness can vary with their model and colour, ML37B gives good results. Orientation is important : the shortest pin must be closer to the PCB edge.
    The LED closer to the PCB edge is ‘packet received’, the other LED is ‘packet sent’.
  • Its name Sponge Board comes from the legendary low IQ of Sponges, animals that don’t even have a brain… Just to say it’s very easy to DIY.

PCB for ULRS Mini PCB with SMD components : the Butterfly board

Warning : there are several user reports about issues with this board. To be on the safe side consider building another of the proposed boards.

Rcgroups user ‘Cereal_Killer’ made a PCB with SMD components, and much smaller than the Spongeboard.

PCB for ULRS Mini with SMD components and RF filter that fits into a JR case : the Warthog Board

User ‘shaggibomb’ from rcgroups did a PCB for Ultimate LRS Mini which will fit in a JR case, has an RF filter and a switching power supply.

  • Schematic of the main board:


  • Schematic of the daughter board:



  • Its name ‘Warthdog’ comes from the favourite animal of the user who designed this board, who lives in Namibia.

The Sami boards : full-featured SMD boards

Rcgroups user Samisin has made three SMD boards with a lot of features :



ULRS Mini SMD board with USB support and RF filter : the Dolphin Board

User ‘artemen’ from rcgroups did a PCB for Ultimate LRS Mini which is very features-rich, includes an FTDI chip, RF filter etc.


  • Its name “Dolphin Board” comes from the extreme intelligence of this animal, in reference to the many features of this board (USB, power management, RF filter etc)
  • Recommended input voltage is max 10V, because it has a linear regulator. A Taranis 2S battery would be perfect.
  • This board is so small it can fit inside a wing (link) :img_20161001_170428img_20161001_171835

Where to find components ?

  • Rcgroups user FlyingW has created a list of links to buy ULRS Mini components here.
  • Digikey or Farnell should also have all the components needed.
  • For the TX, the connector that will receive the RC TX pins is this one:

2016-06-27 22_50_02-0038001395_PCB_RECEPTACLES.pdf

Need a case for your ULRS Mini ?

You can find empty cases here that will plug into your transmitter here. Be careful there are two main types of cases.

  • http://www.horizonhobby.com/transmitter-module-case-jrpa615
  • http://www.horizonhobby.com/integrated-case-for-jr-compatible-air-module-spm6817
  • https://alofthobbies.com/frsky-taranis-module-cover.html


Let me know about your progress building this DIY Ultimate LRS Mini, I’ll publish your PCB or pictures on this page.

Feel free to ask your questions or post your Ultimate LRS Mini on the Ultimate LRS rcgroups thread.

‘Windspeaker’ user from rcgroups forums has made a complete write-up of his experience in building the Ultimate LRS mini, check it out here.


  1. Bandicoot says:

    firstly, love your work.
    Do you think it be prudent or paranoia to add a second RFM23BP, both connected to the mini which then carries out a diversity function?
    I am thinking of the new pixhawk2 cube module that plugs into a board populated with a mini, the RFM23BP (or 2), PSU, and of course some I/O connections (PWMs, SPI, UART). This would reduce external wiring and, I believe, would be a great flight controller/receiver/telemetry unit.
    I am keen to pursue this idea and would be very interested in your thoughts.

    • admin says:

      It’s an excellent idea, given the price of RFM2BP modules it’s cheap to add diversity.

      • Bandicoot says:

        Hi Benoit
        Thanks for the encouragement, I only move ahead because I stand on the shoulders of others.
        Just a few questions if you can find the time.
        1. Does the Mini need to be a 3.3V Version?
        As the RFM23BP specifies 3.3V logic I/O. 5V minis I have used output 5V on the I/O.
        2. Apart from possible restrictions from the question above and assuming same ATMEGA328 pin connections, could I use a Nano in place of the Mini?
        The convenience of in built FTDI for the Tx (Ground) unit is my thinking.
        3. If I were to mount 2 RFM23BPs back to back on a double sided PCB, do you think this would present interference problems. I intend to run the supply to the Rf Mosfets at 9V.
        4. When I do receive the RFM23BPs and build the “Twin transceiver mini ULRS” could you help me with the diversity software?

  2. Axljackass says:

    Is it possible to use Si4432 instead of RFM23b module?any schematic or pin assignments that need to be consider?

    • admin says:

      It’s made for the RFM23BP module, that includes a Si4432, so the firmware should work with a Si4432 but I can’t provide specific support for it. But let me know if it works ?

  3. […] ULRS – darant plokštes oshpark, galima sutilpt su trimis moduliais į < 50$, o atstumo ir galimybių tikrai užteks metams-kitiems. […]

  4. Kirk says:

    Would the ulrs mini be compatible with openlrsng, if so what board type would i select?

    • admin says:

      Hi Kirk, the ULRS mini is not compatible with openlrsng. The pins connected have been chosen to make the easiest possible connection between the RFM module and the Arduino mini.

      • Kirk says:

        Ok, thankyou for the reply. I still have a lot to learn about arduino. So as far as board config is concerned would the difference between brands using atmega328 and RFM23BP is the comunication pins used?

        • admin says:

          Mainly yes. But some pins have different hardware features. For example the hardware SPI is only available on specific pins.

  5. Kent says:

    One benefit that the Orangerx boxes have is the “module” fit into a controller case, like the 9xr or Taranis…
    Would it be possible to install a bluetooth into the case of the controller and have the ground side unit connected to a laptop via FTDI cable?
    The blue tooth is just a passthrough correct?

  6. […] just tested my Ultimate LRS Mini board with a quick arduino sketch, it runs […]

  7. Leif says:


    I’ve just finished soldering my first module (see image-link). PCBs were made at http://dirtypcbs.com/ for 14 USD including shipping and since anything is through the hole is’s simple enough to solder.

    But I have a problem: The arduino is working fine, blinking on port 6 and 7 works. But ULRS is not responding in the new beta console. Parameter saving seems to have no effect. I don’t get an Spectrum analyzer or anything useful in the UI.

    Am I doing something wrong or do I have to supply a PPM signal?



    • admin says:

      This board will only be supported with the 2.24 beta version, that should come very soon now. (No PPM signal is required)

Leave a Reply

Your email address will not be published. Required fields are marked *